* doc/keywords.rdoc: [DOC] Add keywords doc by documenting-ruby/ruby#29
  https://github.com/documenting-ruby/ruby/pull/29 [ci skip]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45023 b2dd03c8-39d4-4d8f-98ff-823fe69b080e